Installing and repairing equipment in the underwater environment can be challenging. Increasingly in the oil and gas industry, hydrocarbon production and processing equipment is installed subsea, and subsea pipelines may be used for transporting hydrocarbon fluids and chemicals to and from such installations. A subsea pipeline may for example be used to transport fluids that have been recovered from the well and carry these away from the well head at an offshore location and over long distance to an onshore facility where the fluids may be processed further.
Hydrocarbon transport pipes are typically formed of steel because this is a cost-effective material. However, this material can corrode if exposed to the fluid contained inside the pipeline. The fluid in the pipeline may contain gas and liquids, including hydrocarbon gas, oil, water and/or brine. Therefore, many pipelines are provided with a thin inner lining or cladding against the steel pipe. The lining or cladding is formed of a material which does not, or is less inclined than the steel to corrode in the presence of the fluid inside the pipeline. The inner cladding is typically formed of a non-corrosive metal material. As a result, many pipelines have a bi-metallic, layered wall structure.
In the course of using a pipeline under water, it can become necessary to make modifications to the pipeline. For example, it may be desirable to join a branch or other equipment, change the layout or replace a section of the pipeline, for example if the section has become damaged or is faulty.
In order to replace or remove a section of a pipeline, the pipe needs to be cut. In the case of a bimetallic pipeline, it is cut through the outer steel layer and the cladding. A concern when installing a new section and joining the pipeline back together is how to maintain the integrity of corrosion resistance across the join.
For single metallic pipelines, options exist to mechanically connect the pipe sections by use of a mechanical connector or to mechanically attach flanges to each end for subsequent joining of the sections using the flanges. A flange may for example be clamped around the pipe end and provided with elastomeric seals between the outer surface of the pipe and the flange. There may be access for fluid to the cut in the cladding, which can be acceptable where corrosion is not an issue.
In the case of bimetallic under water pipelines, a current approach is to cut and recover the pipe ends to the surface and weld the join before replacing the joined pipeline back on the sea bed. This approach avoids exposure of the join to pipeline fluids and allows a full thickness weld to be made that maintains corrosion integrity, but can be inconvenient and difficult under adverse offshore conditions, and it may be impractical in large water depths or remote locations.